1. Field of the Invention
The present invention relates to an optical fiber coupler which is made by arranging first and second single mode optical fibers in parallel, and elongating them upon fusion; and a single mode optical fiber for the optical fiber coupler.
2. Related Background Art
An optical fiber coupler is made by arranging first and second optical fibers in parallel and elongating them upon fusion. FIG. 17 shows a configuration thereof. In the optical fiber coupler 1 shown in FIG. 17, light inputted from a first end 11 of a first optical fiber 10 is power-split in a fusion-elongated part 30 at a splitting ratio corresponding to its wavelength, and thus split light components are outputted from a second end 12 of the first optical fiber 10 and a second end 22 of the second optical fiber 20, respectively. Here, the splitting ratio is 1:0 or 0:1 at some wavelengths. In this case, the light inputted from the first end 11 of the first optical fiber 10 is outputted from one of the second end 12 of the first optical fiber 10 and the second end 22 of the second optical fiber 20. Such an optical fiber coupler 1 is used as an optical splitter for power-splitting light inputted to one terminal and outputting thus split light components from two terminals, an optical multiplexer for multiplexing two wave length bands of light inputted to different terminals and outputting thus multiplexed light from one terminal, an optical demultiplexer for demultiplexing two wavelength bands of light inputted to one terminal and outputting thus demultiplexed light components from different terminals, or the like.
For example, the optical fiber coupler 1 is used as an optical multiplexer in an optical fiber amplifier. In this case, the optical fiber coupler 1 multiplexes the signal light (e.g., in the 1.55-xcexcm wavelength band) to be optically amplified by an optical amplifier medium included in the optical fiber amplifier and the pumping light (e.g., at a wavelength of 0.98xcexcm) to be supplied to the optical amplifier medium. Specifically, when the signal light to be optically amplified is inputted to the first end 11 of the first optical fiber 10 whereas the pumping light outputted from the pumping light source is inputted to the first end 21 of the second optical fiber 20, the optical fiber coupler 1 multiplexes the signal light and pumping light together and outputs thus multiplexed light from the second end 12 of the first optical fiber 10 toward the optical amplifier medium.
The inventors studied the prior art mentioned above and, as a result, have found problems as follows. Namely, it is required for such an optical fiber coupler 1 to minimize its excess loss. A case where light having a power P11 is inputted to the first end 11 of the first optical fiber 10 in the optical fiber coupler 1 will be considered. Here, it is required to minimize the excess loss represented by the following expression:                               Excess          ⁢                      xe2x80x83                    ⁢          Loss          ⁢                      xe2x80x83                    ⁢                      (            dB            )                          =                  "LeftBracketingBar"                                    10              ·                              log                10                                      ⁢                                                            P                  12                                +                                  P                  22                                                            P                11                                              "RightBracketingBar"                                    (        1        )            
where P12 is the power of light outputted from the second end 12 of the first optical fiber 10, and P22 is the power of light outputted from the second end 22 of the second optical fiber 20.
In an ideal optical fiber coupler, as shown in FIG. 18A, the power P12 of light outputted from the second end 12 of the first optical fiber 10 and the power P22 of the light outputted from the second end 22 of the second optical fiber 20 have respective waveforms substantially symmetrical to each other to a horizontal line. Also, the total of the light powers P12 and P22 is substantially equal to the power P11 of the light inputted to the first end 11 of the first optical fiber 10, thereby hardly yielding excess loss.
However, in order to propagate both of two wavelengths of light in a single mode, the cutoff wavelength of optical fibers constituting the optical fiber coupler is set shorter than the wavelength of light on the shorter wavelength side. Therefore, the confinement of light on the longer wavelength side into the core portion becomes weaker, so that the loss of light on the longer wavelength side increases when the optical fiber is bent at a smaller radius of curvature, whereby excess loss is likely to occur. When the confinement of light into the core portion is weak, the power of light is likely to spread outside, whereby the light may partly leak to the outside due to minute variations in the outer form caused by fluctuations in the heat of a heat source (heater or burner) or the like at the time of making the optical fiber coupler. As a result, the output power may fluctuate depending on the wavelength as shown in FIG. 18B. In this case, the total of the power P12 of the light outputted from the second end 12 of the first optical fiber 10 and the power P22 of the light outputted from the second end 22 of the second optical fiber 20 becomes lower than the power P11 of the light inputted to the first end 11 of the first optical fiber 10, thereby yielding excess loss.
In order to reduce the excess loss in view of the foregoing, Japanese Patent Application Laid-Open No. HEI 7-301722, for example, discloses an optical fiber coupler made by using an optical fiber having a so-called double core type refractive index profile. Here, the double core type refractive index profile is one having, successively from the optical axis center, a first core region (with a refractive index n1), a second core region (with a refractive index n2), and a cladding region (with a refractive index n3), wherein the refractive indices have the relationship of n1 greater than n2 greater than n3 in terms of magnitude.
However, though the optical fiber coupler disclosed in the above-mentioned publication intends to reduce the excess loss by using an optical fiber having a double core type refractive index profile, its degree of reduction in excess loss may not be sufficient.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical fiber coupler whose excess loss is fully reduced, and an optical fiber for the optical fiber coupler.
The present invention provides an optical fiber coupler made by arranging first and second single mode optical fibers in parallel, each operating a single mode transmission in a used all wavelength region, and elongating the first and second single mode optical fibers upon fusion; wherein each of the first and second single mode optical fibers has a cladding portion, placed about a core portion, having a refractive index gradually decreasing outward in a radial direction.
In the optical fiber coupler, since the refractive index of the cladding part of each of the first and second single mode optical fibers gradually decreases outward in the radial direction, the light spread to marginal areas of the core portion can return to the core portion without being emitted to the outside even when influenced by disturbances such as minute bending, whereby the excess loss can fully be reduced.
Also, the present invention provides an optical fiber coupler made by arranging first and second single mode optical fibers in parallel, each operating a single mode transmission in a used all wavelength region, and elongating the first and second single mode optical fibers upon fusion; wherein, in each of the first and second single mode optical fibers, letting r be the radial distance from the optical axis center, xcex94n(r) be the relative refractive index difference at the position r within a core portion with reference to the refractive index of a cladding portion placed about the core portion, xcex94npeak be the peak value of the relative refractive index difference xcex94n(r) at the position rpeak, and a be the core radius, the relative refractive index difference xcex94n(r) satisfies the relationship of xcex94n(r)xe2x89xa6xcex94npeak[1xe2x88x92(r/a)3] in the range of rpeakxe2x89xa6rxe2x89xa6a.
In this optical fiber coupler, since the core portion of each of the first and second single mode optical fibers has such a profile, the bending loss decreases, so that the excess loss can be reduced sufficiently.
Preferably, in this optical fiber coupler, the refractive index of the cladding portion of each of the first and second single mode optical fibers gradually decreases outward in a radial direction.
In this optical fiber coupler, since the light spread to marginal areas of the core portion can return to the core portion without being emitted to the outside even when influenced by disturbances such as minute bending, the excess loss can fully be reduced.
In the optical fiber coupler in accordance with the present invention, the decrease in refractive index of the cladding portion in each of the first and second single mode optical fibers may be such that a predetermined region in the radial direction has a greater rate of decrease than that on the inner and outer sides thereof. This makes it possible for the cladding portion to be constructed by a plurality of stages.
Preferably, in the optical fiber coupler in accordance with the present invention, the difference between the average refractive index and minimum refractive index in a transverse cross section of the cladding portion is 0.02% or less in each of the first and second single mode optical fibers. This is because of the fact that, if the difference exceeds 0.02%, then a multi-mode tends to occur in the cladding portion due to its refractive index distribution, which may be converted into a higher-order mode when the single mode optical fibers are processed into the optical fiber coupler, whereby excess loss is likely to occur.
In the optical fiber coupler in accordance with the present invention, the refractive index distribution of the cladding portion in each of the first and second single mode optical fibers may be formed by a concentration distribution of at least one of chlorine atom, GeO2, and OH group in SiO2 glass. Alternatively, the refractive index distribution of the cladding portion may be formed by a distribution of an internal distortion imparted to the optical fiber upon drawing thereof. This makes it possible to easily realize the above-mentioned refractive index distribution of the cladding portion.
In the optical fiber coupler in accordance with the present invention, the relative refractive index difference xcex94n(r) may satisfy the relationship of xcex94npeak[1xe2x88x92(r/a)]xe2x89xa6xcex94n(r)xe2x89xa6xcex94npeak[1xe2x88x92(r/a)2.5] in the range of rpeakxe2x89xa6rxe2x89xa6a. This enables each of the first and second single mode optical fibers to further reduce the bending loss, whereby the excess loss can further be lowered.
In the optical fiber coupler in accordance with the present invention, the relative refractive index difference xcex94n(r) may attain the peak value xcex94npeak at a distance r within the range of 0xe2x89xa6rxe2x89xa6a/2. This can reduce the loss at the time of making a coupler caused by the influence of the recess at the core portion center in each fiber.
In the optical fiber coupler in accordance with the present invention, each of the first and second single mode optical fibers may have a cutoff wavelength xcexc shorter by at least 300 nm than the longest wavelength in use. In this range, the effect of reducing the excess loss at the time of making a coupler is greater than that of conventionally known fibers having a step type refractive index distribution.
In the optical fiber coupler in accordance with the present invention, each of the first and second single mode optical fibers may have a cutoff wavelength xcexc shorter than 980 nm. This makes it possible to form a coupler whose excess loss is lowered in a multi/demultiplexer for a wavelength of 980 nm and the band of 1500 to 1610 nm, as with an erbium-doped fiber type amplifier.
When the bending loss of each of the first and second single mode optical fibers in the wavelength band of 1.50 xcexcm to 1.65 xcexcm is less than 1 dB/m at a bending radius of 15 mm, the optical fiber coupler in accordance with the present invention acts more effectively. This enables the first and second single mode optical fibers to lower their respective bending losses, whereby the excess loss can fully be reduced. It is also advantageous in reducing the curvature of pigtail parts when accommodating the coupler.
The present invention provides an optical fiber for an optical fiber coupler, which is suitably usable as the optical fiber coupler in accordance with the present invention.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention.